After a bit of a think, I have a solution that will work without the addition of any extra circuitry, I just have to change the specification/expectations of the circuit. To explain: at the moment I have these input options:
1) 1:1 meaning direct input to the LM2917 of 1 pulse per revolution, bypassing the 4018. This is the 'problem' input and will only work at top RPMs if the reflected light dwells on the photodiode for at least ¼ of a revolution, so reflection off the face of a nut is out and therefore some means of preparing the rotating body so that a ¼ of it's surface is shiny to reflect the laser light has to be carried out.
2) All the other input options 2:1( 2 pulses per rev ), 6:1, 8:1, 10:1, all go through the 4018 that latches the short duration pulses from the face of the nut and also provides the 1 input pulse per rev required by the LM2917 as it stands in my circuit.
Given the limitation of the 4018 in terms of options as a divider, my initial choice of user input options ( 2:1, 6:1, 8:1, 10:1 ) was arbitratry in that I simply chose those that I thought may be the most useful, i.e. to make my circuit the easiest to apply to a number of different types rotating bodies ( in particular the 6:1 option ).
The solution to my N.o. 1) input 'problem' is simply to change the input options and have them all go through the 4018. Now, a 1 pulse per revolution input is always fed through the 2:1 input of the 4018 giving effectively a ½ pulse per revolution feed into the LM2917. All I have to do now is double the output voltage of the LM2917 by changing the R1C1 combination. This scheme now affects the variety of user input options and limits them to 2:1, 3:1, 4:1 and 5:1 by using the 4:1, 6:1, 8:1 and 10:1 divider options respectively of the 4018. The most useful of these will always be the 2:1 option and to a lesser extent the 3:1 input option, so this change of scheme is something I can happily live with..!